FREE APPENDAGES — MAMMALS 



297 







Sg 



with the axis of the limb, a condition largely retained in Hyracoids and Probos- 

 cidians (fig. 32s). Diplarthrous tarsi have a strong tendency towards alterna- 

 tion of the elements of the two rows so that each proximal bone articulates with 

 two of the distals, as in all recent true Ungulates. 



The common features of all are a short stout humerus with large external 

 tuberosities and no ulnar foramen; ulna and fibula incomplete; carpus and tarsus 

 diplarthrous, the talus articulating with central (navicular) and calcaneus, the 

 centrale with carpalia 2 and 3. The femur has the head separated from the 

 shaft by a neck. The poUex is greatly reduced or absent; a well-developed 

 patella is present. The recent species fall into two lines, Perissodactyla and 

 Artiodactyla (fig. 320). In the first the femur has a third trochanter and the 

 axis of the limb passes through digit 3, around which 

 the other digits are symmetrically arranged. Artio- 

 dactyls lack the third trochanter and have the pedal 

 axis running between digits 3 and 4, which are about 

 equal. From the prominence of digit 3 in Perisso- 

 dactyls, of 3 and 4 in the other group, these toes 

 support most of the body weight, the lateral ones 

 tending to degenerate, the extreme of this being in 

 modern horses where only 3 persists, except as repre- 

 sented by the greatly reduced metacarpals and meta- 

 tarsals; and in Tylopoda where only digits 3 and 4 

 are retained. 



The extinct orders of Ungulates are largely based 

 on foot structure, and usually have taxeopodous 

 feet, these occurring in all feet of Ancylopoda, 

 Condylarthra, Litopterna and Amblypoda and at 

 least in the tarsus of Toxodontia and the carpus of 

 Typotheriidae, the latter family having the ente- 

 picondylar foramen. A few have the third 

 trochanter. 



In Perissodactyla (fig. 321) the head of the 

 humerus projects shghtly, the tuberosities are 

 short and strong, and its distal end is transversely truncate. Ulna and 

 radius vary with the foot structure; the radius, large at both ends, is constant; 

 the ulna being well developed and separate in tapirs and Rhinoceros; Mac- 

 rauchenia has the two bones fused and in horses the ulna persists in the ole- 

 cranon and a tapering part fused with the radius. The third digit is always the 

 strongest, and with its preeminence the carpal bones are modified in correlation 

 with the reduction of the digits, but radiale and intermedium do not unite. The 

 modifications are most marked in the lateral bones. Hippopotamus has digits 

 2 and 4 nearly as large as 3, and Ta pirns has 5 large, but not functional. Other 

 genera show a progressive reduction of the lateral digits (evident in the history of 

 the horse) until in modern Equus the third digit alone is functional and all that 

 remains of the lateral digits are the pair of splint bones at the upper end of the 



qc^ 



of 



Fig. 320. — Schema 

 bones in manus of {A) a 

 Perissodactyl (horse) and 

 (B) Artiodactyl (cow), 

 (Weber, '04). The shaded 

 parts are those retained 

 from the primitive five dig- 

 its; the straight line is the 

 axis of the appendage, i, 

 intermedium; r, radiale; u, 

 ulnare, 1-5, carpalia; I-V, 

 digits. 



